Trans harness

Good evening everyone.
Haven't posted here in a long while and the truck has been sitting a while.

So I have a 99 Tahoe 2wd. Truck drives until the 2-3 shift and then free revs. Sometimes it'll pop into 3rd but that's rare. Changed the filter, shift solenoids, fresh oil, and gaskets. Found the trans is not the original trans but rather came from an early 90s truck. The harness has been chopped up and older style connections have been rigged to make it work.

Big question is, rather than fight to fix all the messed up and exposed wiring, where can I source a good harness and is it worth it.